Current MOT tests only require mechanics to look to see if there is a DPF present – they do not need to test to see if it works

As there is no physical test of the filter required, a faulty part could easily pass through an MOT, or simply be removed once their car has been deemed roadworthy.

But anyone found guilty of driving a diesel vehicle without a DPF could be slapped with a £1,000 fine for a car, and as much as £2,500 in penalties for a van.

All diesels produced after 2009 are legally required to have a filter to reduce their pollution levels.

The Driver and Vehicle Standards Agency claims just 1,800 drivers have been caught without a DPF in their car since 2014, but an investigation by BBC 5 Live has found the number of cars without a filter could actually be significantly higher.